The present invention relates to a device for Monitoring the speed of rotation of an electric motor.
It applies in particular to the monitoring of the speed of rotation of the rotor of a centrifuging apparatus so as to prevent the latter from being spun at a speed of rotation above its maximum allowable value.
To improve the reliability of such devices, they are often fitted with redundant monitoring units so as to prevent the failure of one of them from having harmful consequences for the motor.
Thus, there is known from the state of the art, a device for monitoring the speed of rotation of an electric motor, of the type comprising a unit for controlling the speed of rotation of the motor and an analogue speed limitation unit linked to the control unit and including means for comparing between at least one monitoring signal representative of the speed of rotation of the motor imposed on the latter by the control unit and a threshold value corresponding to the maximum speed at which the motor is permitted to operate, and means for at least temporarily disconnecting the motor from its supply should the threshold value be exceeded.
A device of this type therefore makes it possible to prevent the motor from being spun at a speed of rotation above its maximum allowable speed.
The objective of the invention is to provide a monitoring device of this type, which is furthermore capable of preventing the motor from being rotated should the speed limitation unit malfunction.
Its subject is therefore a monitoring device of the aforesaid type, characterized in that prior to each startup of the motor, the control unit carries out a phase of testing the correct operation of the speed limitation unit by generating a drive signal corresponding to a speed of rotation value greater than the threshold value.
The monitoring device according to the invention can furthermore include one or more of the following characteristics, taken in isolation or according to all the technically possible combinations:
it furthermore includes second means for disconnecting the motor from its supply during the test phase;
the monitoring signal is formulated from an output signal from a comparator of a speed regulation loop which receives as input a preset signal delivered by the control unit and a measurement signal for the speed of rotation of the motor;
the control unit is linked to the output of the first means so as to disconnect the motor with a view to detecting the correct operation of the speed limitation unit;
the control unit consists of a microprocessor-based control unit delivering motor drive signals in the form of pulse width modulation control signals fed to a three-phase electrical energy supply stage for the motor, the said supply stage being supplied from a DC voltage source by way of a variable-impedance stage under the control of the speed limitation unit, and the said at least one monitoring signal is formulated from the drive signals; and
the variable-impedance stage consists of a filtering stage comprising a resistive element in parallel with which is linked a bypass circuit equipped with a switching element which is opened by the comparison means, should the threshold value be exceeded.
The subject of the invention is also a centrifuging apparatus comprising a rotor driven in rotation by an electric motor, characterized in that it is equipped with a device for monitoring the speed of rotation of the motor as defined above.